Summary of Significant Accounting Policies - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	$ 31.4	$ 28.9	$ 30.3	$ 24.9
Provision for expected credit losses	5.2	6.6	6.7	10.7
Write-offs, net of recoveries	(1.5)	(1.1)	(1.9)	(1.2)
Allowance at end of period	35.1	34.4	35.1	34.4
Spire Missouri
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	23.8	21.4	22.6	18.1
Provision for expected credit losses	4.7	5.7	6.8	9.0
Write-offs, net of recoveries	(1.5)	(0.5)	(2.4)	(0.5)
Allowance at end of period	27.0	26.6	27.0	26.6
Spire Alabama
Accounts Notes And Loans Receivable [Line Items]
Allowance at beginning of period	6.7	6.2	6.6	5.5
Provision for expected credit losses	0.5	0.6		1.3
Write-offs, net of recoveries	(0.1)	(0.3)	0.5	(0.3)
Allowance at end of period	$ 7.1	$ 6.5	$ 7.1	$ 6.5